1. Field of the Invention
The present invention relates to a perforator, more particularly to a perforator for forming holes in metal plate material, such as a presensitized plate, which will be durable in use.
2. Description of the Prior Art
A lithographic printing system is generally operated by utilizing a preseuisitized plate (herein referred to as a PS plate), which comprises a support consisting of a thin metal plate of e.g. aluminum or steel. Such a PS plate processed for lithography is mounted in a printer. To position the PS plate precisely in the printer, the PS plate is provided with punched holes to receive positioning members.
In a manufacturing process for such PS plates, a perforator is used for punching the plate material. The perforator is a movable blade or punch shaped to punch a hole in order to pierce the plate material, and a stationary blade or die for slidably receiving the punch. The plate material is continuous or is a separate piece and is sandwiched between the punch and the die so as to punch holes in the plate material. Such a perforator is usable to punch simultaneously plural superposed pieces of material.
Good formation of punch holes which will be stable even after long use requires high quality of the punch and die of the perforator: the punch and die should be sufficiently hard, should have each blade precisely constructed, and should have sufficiently small roughness on the faces of the blades. It is usual to form the punch from high speed steel SKH, and to form the die from special tool steel SKD, and to set the roughness on the blade faces to be 20.0 .mu.m, preferably as small as 1.0 .mu.m. It is general to provide clearance between the punch and the die, of 5 to 8% of the thickness of the plate material to be punched.
In the course of repeated punching, finely powdered aluminum dust is generated from the plate material. The fine dust sticks on the blade faces, degrades the sharpness of the punching structure, and causes the punched edges to have irregularities, which are raised over the plate surface by contact with the punches when the punches are raised and removed from the punch holes. In view of this problem, it is proposed in Japanese Patent Laid-Open Publ. No. 61-241096 to superpose the metal plate material on light-shielding polyethylene-laminated lining paper and to punch the plate material from the side of the lining paper. The use of the polyethylene-laminated lining paper is somewhat effective in maintaining the sharpness of the punching structure, because the lining paper can wipe the fine dust off the blade faces.
Widespread use of polyethylene-laminated lining paper, however, could be harmful when discarded as industrial waste. Moreover, the mass production of PS plates can be counterproductive, in view of the public concern now shown for protection of the global environment against destruction caused by considerable wastes.
It might be better, for protecting PS plates from ambient light, to use polyethylene lining paper superposed on the PS plates. An experiment was conducted with thin aluminum plate with which polyethylene lining paper was used and which is 150 .mu.m thick. The SKH-formed punches and/or the SKD-formed die had a roughness of 1.0 to 2.0 .mu.m. A round punch hole 2 formed by the punches of a sheet perforation was 4 mm across, as illustrated in FIG. 10. In FIG. 11, a slot-like punch hole 3, formed by punches of a web perforator moving in the arrowed direction, was 10 mm long and 6 mm wide.
It has been observed that 10 to 20 times of operation of punching the plate material resulted in generation of an unwanted rise 5 or fold 6 around punch holes 2 and 3, because irregularities inside the punch holes 2, 3 are raised by the punches upon being retracted from the punch holes 2, 3. This lowers the quality of PS plates as products.
The use of such conventional perforators, after every 10 to 20 punching operations, requires inspection or cleaning of the blade faces. A problem lies in that there is a considerable limit to improving efficiency in punching out plate material.